After an item has been scraped by a spider, it is sent to the Item Pipeline
which process it through several components that are executed sequentially.

Each item pipeline component (sometimes referred as just “Item Pipeline”) is a
Python class that implements a simple method. They receive an Item and perform
an action over it, also deciding if the Item should continue through the
pipeline or be dropped and no longer processed.

This method is called for every item pipeline component and must either return
a Item (or any descendant class) object or raise a
DropItem exception. Dropped items are no longer
processed by further pipeline components.

Let’s take a look at the following hypothetical pipeline that adjusts the price
attribute for those items that do not include VAT (price_excludes_vat
attribute), and drops those items which don’t contain a price: